Back to water quality

Using hydrological optics to study seagrassesPhotograph of pondweed on surface of water.

The optical properties of water determine the color and clarity of the water, which influence the amount of light that reaches the bottom of a water body. This light penetration then determines the areas in which seagrasses, or more generally, submerged aquatic vegetation (SAV), can live. SAV applies to both freshwater and marine seagrasses. Today the area where SAVs can survive in coastal regions is decreasing as a result of increasing turbidity or murkiness, causing seagrasses to die where they once flourished.  

Seagrasses are not altogether different from terrestrial plants. They look similar and like terrestrial plants they need light for photosynthesis. This light requirement means that seagrasses can only live in relatively shallow water,where an adequate amount of light can reach the bottom.

For this reason seagrasses typically live in and define the littoral zone, or the edge waters, of a body of water. This zone typically corresponds with the Secchi disk depth as shown below. 

Diagram of littoral zone and secchi disk depth, which corresponds to the depth where seagrass lives.

Back to top
Hydrologic optics looks at the factors that determine the color and clarity of water which determines where seagrasses can live. As evident in Chesapeake Bay, many seagrass communities have disappeared as a result of a negative change in water quality, which reduced the available light. Estuarine water quality monitoring programs at SERC will help in the general decision- making process for management strategies for the bay. Reductions in colored dissolved organic matter, phytoplankton and suspended particulate matter may be necessary for seagrass regrowth and survival. These components affect the optical properties of the water which determine water quality.

Seagrasses require a minimal amount of light to survive, which has been determined for many species. In general, average minimal light requirements are: 4-29% of the total light measured just below the surface, usually around 20%

In comparison, terrestrial plants require significantly less light, 0.5-2% of the light available at the top of the tree canopy. Phytoplankton have even lower light requirements, 0.0005-3% of the light just below the surface. The lower light requirements of phytoplankton is good news, because they are the bottom of the food chain, providing food for all other organisms. On the other hand, phytoplankton can be part of the equation that limits or prevents the growth and survival of SAV. Their pigments can block light from reaching SAVs and eventually reduce the oxygen concentration of the water as they decay. Both lower light and lower oxygen levels decrease the chances of SAV survival.

The high minimal light requirements of seagrasses compared to terrestrial plants and plankton underlines the importance of water transparency for seagrasses, which are on the borderline of local extinction in the Rhode River and the Chesapeake Bay.

We have a partial list and description of the seagrasses of Chesapeake Bay.

Seagrasses are important because they provide:

  • energy for the aquatic environment in the form of food
  • habitat for adult and juvenile fish, invertebrates, waterfowl and shellfish which can mean protection from predators, food and a breeding ground
  • a protection and incubation area for larvae of many species
  • foliage that acts as a sediment trap, therefore increasing sedimentation, reducing turbidity, increasing water clarity and potentially reducing water pollution
  • erosion protection by reducing the focus of wave action and the energy of tidal currents

Therefore the survival of seagrasses is important for both water quality and marine life.

Back to top

Smithsonian Environmental Research Center
647 Contees Wharf Road, P.O. Box 28, Edgewater, MD 21037-0028
Phone: 443-482-2200
Fax: 443-482-2380
All pictures are borrowed from Spenser-Jones, David and Max Wade. Aquatic Plants (1986). ICI Professional Products; Surrey.